1. Field of the Invention
The invention relates to an inherent control mechanism for direct drive reversible hydraulic pumps supplying power to hydraulic circuits performing work, and in particular, a dual, coupled check valve for eliminating the necessity for pressure relief valves for resolving excess flow in a discharge leg of any hydraulic circuit such as coupled dual motor, reversible hydraulic drive systems, powered by a direct drive, reversible hydraulic power source.
2. Description of the Prior Art
In U.S. Pat. Nos. 5,184,357, 5,327,590, and 5,546,751 Applicant, Harry J. Last describes dual motor, reversible hydraulic drive systems in combination with a reversible source of hydraulic power for covering and uncovering swimming pools with flexible sheets of fabric as safety covers. One of the hydraulic motors is coupled to a cover drum for retracting, winding the cover around the cover drum. The other hydraulic motor is coupled to a cable reel for winding up cable attached to the front of the cover for extending the cover across the swimming pool. While these invented systems perform quite satisfactorily using conventional remote reversible hydraulic power packs and/or remote unidirectional hydraulic pump in a combination where a solenoid valve is utilized for reversing the direction of liquid flow to the dual motor drive system, excess discharge problems arise when simpler direct drive, reversible hydraulic power packs/pumps are chosen as the source of hydraulic power.
For example, a typical hydraulic gear-type pump schematically illustrated in FIG. 1, in cross section, can be used as a single direction unit in combination with a three position solenoid valve means for reverse direction of liquid flow. The disadvantage of such systems is the necessity for the three position solenoid valve which directs hydraulic flow in a first direction in the particular hydraulic at position one, recycles hydraulic liquid to the pump input port at position two, and directs hydraulic flow in a opposite direction in the particular hydraulic at position three. Such solenoid valves typically electrically fail due to wear and environmental conditions.
Alternatively, rotation of the intermeshing gears in the typical hydraulic gear-type pump can be reversed for reversing the direction of liquid flow in a connected hydraulic circuit. In particular, if gear 13(a) is rotated counterclockwise, gear 13(b) rotates clockwise, and pumping hydraulic liquid from port 15, around the periphery of the housing and the gear teeth and out port 14. Similarly if gear 13b is rotated counter clockwise hydraulic, gear 13 a rotates clockwise pumping liquid from port 14 out port 15.
In particular, with reference to FIG. 2, a typical hydraulic circuit is schematically illustrated that incorporates a direct drive, reversible hydraulic pump 16 for extending and retracting a typical hydraulic cylinder typically that includes:                (i) a hydraulic cylinder HC where the volume on the rod side leg of the circuit 30 per unit cylinder length is less than the volume per unit cylinder length on the blind side leg 31 of the hydraulic circuit (because of the rod);        (ii) a reversible pump 16 with inlet/output ports 16a & 16b forming a hydraulic circuit with the rod side leg 30 and blind side leg 31 of the hydraulic cylinder HC;        (iii) check valves 8 & 9 hydraulically coupling the respective input/output ports 16a & 16b of the reversible pump 16 to the hydraulic liquid reservoir;        (iv) pressure relief valves 11 &12 relieving pressure above a set point on the blind side leg 31 and rod side leg 30 of the of hydraulic circuit.        
Because, the volume per unit length of the rod side leg 30 is less than the volume per unit cylinder length of the blind side leg 31 of the hydraulic circuit, when the system circulates liquid for the translating piston into the blind side leg 31 of hydraulic cylinder HC (indicated by arrow A), for each unit volume V of liquid pumped (output) into the rod side leg 31, that unit volume V plus increment volume ΔV is output into the blind side leg 30 of the hydraulic circuit.
Since the hydraulic circuits typically are maintained liquid full by check valves such as 8 & 9, as the piston translates into the blind side leg 31 of the hydraulic circuit check valve 8 immediately closes, and pressure increases. The reversible motor 16 continues to pump seeing the increase in liquid pressure load due to the incremental increases in volume output ΔV into the blind side of the hydraulic circuit both at its input/output port 16b on the rod side leg of the hydraulic circuit and at its input/output port 16a on the blind side leg. If the reversible pump 16 is lossy allowing flow through from the blind side leg to the rod side leg of the hydraulic circuit, pressure relief valve 12 will ultimately release stopping translation of the piston. If the reversible pump is not lossy, pressure relief valve 13 will release, and the piston will continue to translate requiring the reversible pump to expend sufficient energy necessary to pump liquid into the rod side leg of the circuit against the pressure set by pressure relief valve 11 rather than at reservoir pressure.
Similar excess liquid volume problems are encountered when a direct drive, reversible hydraulic gear-type power source is incorporated into a combination of drive coupled, anal hydraulic reversible hydraulic motors with mechanically coupled drives for driving winding systems translating a structure such as a swimming pool cover back and forth across a swimming pool. In such winding systems the rotational rate of the driving motor is constant while rotation rate of the driven motor (functioning as a pump) varies. For example, in the pool cover systems described in the Applicant's prior patents (supra) when the cover is being driven for winding the around the cover drum it rotates at a constant rate, but the rate at a which cover winds increases or accelerates as the circumference of the drum and wound up cover increases. This acceleration in cover winding rate, in combination with the acceleration caused by the decrease in circumference of the unwinding cable reel(s) continuously accelerates the rotational rate of the driven (pumping) motor.
In such winding systems, initially the liquid volume output from the driving motor typically exceeds the input volume requirements of the driven (pumping) motor, which if not alleviated causes the driven motor to rotate at a faster rate possibly overdriving, and unwinding of the component coupled to its drive shaft allowing slack to develop between the translating mechanical components coupling to the respective mechanical drive shafts of the coupled hydraulic motors. In U.S. Pat. No. 5,546,751, the Applicant utilizes an anti-cavitation manifold to direct such excess liquid to the return hydraulic line for the reversible source of hydraulic power. [See Col 8 lines 35–55.] Then at the point, where the pump demand of the driven motor equals then exceeds that of the output from the driving motor i.e., the point where its liquid output is not sufficient to supply input demanded by the driven (pumping) motor, the anti-cavitation manifold couples the output from the driven motor to its input to prevent cavitation. [See Col 8 lines 55–62.]
The Applicant recognizes in U.S. Pat. No. 5,546,751 [Col. 9, ll. 20–41] that the invented anti-cavitation manifold in static circumstances, does not prevent the drive shaft of the last driving motor from rotating in the driving direction responsive to excess volume in the particular hydraulic supply line from the reversible hydraulic power source, e.g. liquid pushed out from the hydraulic lines connecting to the pressure relief valve monitoring and preventing over pressurization in the liquid volume input leg of the hydraulic circuit. An external force can also cause over rotation of the particular driving motor moving the translating components coupling between the respective mechanical drives of the dual reversible hydraulic motors to an undesired position particularly since, in static circumstances the anti-cavitation manifold allows drive shaft of the last driven (pumping) motor to rotate in either the ‘driving’ or ‘pumping’ direction responsive to an external force.
A prior art solution to the incremental excess volume problem in hydraulic circuits driven by direct drive reversible power describes a shuttle or control spool valve shuttling in a passageway in module, responsive to hydraulic volume increases and decreases in hydraulic liquid volume in the respective legs of the circuit, functionally similar to the duel shuttle ball-shuttle rod and translation passage combination of Applicant's anti-cavitation manifold (U.S. Pat. No. 5,546,751 Col. 2, l. 63–Col. 4, l. 21). The translating shuttle or spool of the valve functionally isolates the high pressure leg of hydraulic from the low pressure leg and redirects all liquid flow from the low pressure leg of the hydraulic circuit directly to the reservoir bypassing the particular check valve 8 or 9 maintaining the system liquid full. [See Oildyne Brochure]
The disadvantages of the Oildyne shuttle cock or spool value, relate to the fact that the entire flow in the particular discharge leg of circuit is directed into reservoir requiring the direct drive, reversible motor to make up or pump hydraulic all liquid requirements for driving the circuit from the reservoir via check valves 8 or 9 which must be present for proper functioning of any involved hydraulic circuit.